Solitary Fibrous Tumour of Bone

Overview

Terminology and Classification

• The term hemangiopericytoma (HPC) has been unified with SFT since both share the defining NAB2::STAT6 gene fusion on chromosome 12q13.
• Under the 2020 WHO classification, extrameningeal SFT (including bone) is listed as intermediate (rarely metastasizing) or malignant.
• For bone specifically, the WHO 2020 update recognizes intraosseous SFT as a primary bone tumour category.

Molecular Biology - The Defining Alteration

• NAB2::STAT6 fusion is found in ~83-100% of cases (RT-PCR or NGS)
• The most common fusion variants are NAB2ex4-STAT6ex2 and NAB2ex6-STAT6ex16/17
• The fusion converts NAB2 (a transcriptional repressor of EGR1) into a transcriptional activator, driving oncogenesis via EGR1 target genes
• The chimeric STAT6 protein translocates to the cell nucleus, which is detectable by IHC - this is the basis of the diagnostic STAT6 nuclear staining

Epidemiology

Clinical Presentation

• Often asymptomatic until large or until pathological fracture occurs
• May present with pain, swelling, or an incidental radiological finding
• Pathological fracture can be the presenting event (as in the case of proximal femur intertrochanteric SFT)
• Paraneoplastic hypoglycaemia (Doege-Potter syndrome) - due to IGF-2 secretion - is rare but reported even with bone primaries
• Bone SFTs tend to be slowly growing but can behave aggressively independent of conventional histological parameters

Imaging

Plain Radiograph / CT

• Lytic, well-demarcated intramedullary lesion
• May show cortical thinning, expansion, or destruction
• Calcifications occasionally present
• No specific periosteal reaction is typical
• Differential on imaging is broad: Ewing sarcoma, fibrosarcoma, GCT, metastasis, plasmacytoma

MRI

• Heterogeneous signal - fibrous areas hypointense on T2, vascular areas enhance avidly
• Rich vascularisation produces prominent contrast enhancement
• Helps define soft tissue extension

Gross Pathology

Histopathology

Classic Features

• "Patternless pattern" - haphazardly arranged spindle-to-oval cells with indistinct cell borders
• Dense collagen bands admixed with the tumour cells
• Characteristic branching, staghorn (haemangiopericytoma-like) thin-walled vessels - ectatic, ramifying
• Finely dispersed chromatin; cells have plump or tapered nuclei with lightly eosinophilic cytoplasm
• Alternating hypercellular and hypocellular areas

Malignant Features (any 2 of the following raise concern)

• High cellularity with cytologic atypia
• Mitotic activity >4 mitoses/10 HPF
• Necrosis
• Invasive growth pattern

Immunohistochemistry

Molecular Confirmation

• RT-PCR for NAB2::STAT6 fusion transcripts - sensitive but requires knowledge of which exons are fused
• Next-generation sequencing (NGS) / RNA sequencing - preferred for comprehensive fusion detection
• FISH - can detect rearrangement at 12q13 but does not identify the specific fusion partner

Diagnosis - Key Differential

Treatment

Surgery

• Wide/en-bloc resection with negative margins is the mainstay for localised disease
• Intralesional or marginal resection carries high recurrence risk
• For bone: may involve prosthetic replacement (e.g., proximal femur resection with endoprosthesis, as in reported cases)
• Segmental resection (e.g., mandibulectomy) for jaw SFTs

Radiation

• Adjuvant radiotherapy is recommended for positive or close margins
• Definitive radiotherapy is an option for unresectable lesions and has shown promise for local control
• Fractionated RT is standard when used post-operatively

Systemic Therapy (for advanced/metastatic disease)

• Anthracycline-based chemotherapy (doxorubicin ± ifosfamide) as first-line
• Antiangiogenic agents: pazopanib, sunitinib, sorafenib - increasingly used given the rich vascular nature of SFT; considered after first-line failure
• Imatinib, axitinib, eribulin under investigation
• No established standard for advanced disease; enrolment in clinical trials is recommended

Prognosis

• Pleural/soft tissue SFTs: ~90% behave indolently; ~10% show aggressive course
• Primary bone SFTs behave more aggressively: the Bianchi 2021 series reported disease-specific survival of 64% at 5 years and 49% at 10 years
• Unlike soft-tissue SFTs, histological grade and mitotic count do NOT reliably predict outcome in bone SFTs
• Risk factors for recurrence include incomplete resection and late metastasis (bone, liver, lung are metastatic targets)
• Intracranial SFT/HPC has a 60-80% local recurrence rate and 25% rate of systemic metastasis, predominantly to bone and liver

Key Points Summary

1. Primary intraosseous SFT is extremely rare and frequently misdiagnosed (Ewing sarcoma is a common initial misdiagnosis)
2. The NAB2::STAT6 fusion on 12q13 is pathognomonic - confirmed by RT-PCR, NGS, or nuclear STAT6 IHC
3. CD34+ / STAT6 nuclear+ immunoprofile is diagnostic
4. Bone SFTs have a more aggressive clinical course than soft tissue counterparts, and standard soft-tissue grading criteria are not reliably prognostic in bone
5. Wide surgical resection with clear margins is the most important determinant of outcome
6. Adjuvant/definitive radiotherapy and antiangiogenic agents are useful in unresectable or metastatic settings
7. Long-term follow-up (>10 years) is mandatory given late recurrences and metastases

• Robbins, Cotran & Kumar Pathologic Basis of Disease - Solitary Fibrous Tumor
• Murray & Nadel's Textbook of Respiratory Medicine - SOLITARY FIBROUS TUMOR
• Bradley and Daroff's Neurology in Clinical Practice - Hemangiopericytoma and SFT
• Bianchi et al. (2021). Clinical, Histological, and Molecular Features of Solitary Fibrous Tumor of Bone. Cancers. PMID 34069378
• Ren et al. (2024). Advances in the molecular biology of SFT. Cancer Metastasis Rev. PMID 39120790
• Janik et al. (2023). Diagnostics and Treatment of Extrameningeal SFTs. Cancers. PMID 38136399
• Argyris et al. (2024). Primary Intraosseous SFT of the Mandible. Head Neck Pathol. PMID 39621174
• Coppola et al. (2022). Primary intraosseous SFT - case report and review. Pathologica. PMID 36305024